The transportation of refrigerated products (i.e. refrigerated articles and/or compositions) is generally accomplished by storage of the product in a refrigerated container. The most common method of cooling a refrigerated container is the use of a recirculating refrigeration unit which recirculates a refrigerant gas, e.g., a Freon chlorofluorocarbon, through a direct expansion refrigeration cycle. Such units are generally driven by an outside power source, e.g., power takeoff from the tractor of a tractor trailer, or by a dedicated self-contained power source, e.g., a small gasoline or diesel engine or electric motor.
Another method of cooling used in the transport of refrigerated products is by the use of a container fitted with cold plates. The cold plates are filled with a eutectic solution, e.g. a brine, that is refrigerated and frozen when the container is connected to a refrigeration system at a terminal. While this system is mechanically simple and dependable, the system is inefficient and does not have the flexibility of a recirculation system.
A third system used in refrigeration of transportation containers are cryogenic systems which use a reserve of super-cooled gas, e.g., liquid nitrogen or carbon dioxide, as a refrigerant. The super-cooled, i.e. cryogenic, gas is allowed to expand, e.g., by spraying into the interior of the container, and thereby cool the container.
While the rapid cooling to very low temperatures provided by the cryogenic system may be desirable, it has been found the weight and bulk of the cryogenic refrigerant reserve transported with the container often makes the system unfeasible, especially where great refrigerant demands are encountered. While the recirculation refrigeration units commonly used are much less bulky, they are not as efficient at quickly cooling the container to refrigeration temperatures after exposure of the interior to an ambient atmosphere. Such quick cooling, i.e. rapid pull down, is especially useful in preventing thawing of a frozen food. In particular, in a distribution system where the container is opened a number of times for removal of only a portion of a transported, frozen load, the commonly used recirculation units may allow a significant portion of the load to thaw before it is delivered.